The present invention relates generally to vacuum gripping devices and, more particularly to self-actuating vacuum extractors and locators for handling electronic parts and/or crystalline wafers.
Workholders and extractors have previously employed vacuum devices to hold and handle relatively small or lightweight workpieces. These vacuum devices typically employ some form of gripping orifice which is connected to a fluid sink or low pressure source, such as a vacuum pump. When a portion of the workpiece approaches this orifice, the pressure differential between the orifice and the surrounding atmosphere beyond the workpiece generates a suction force which retains the workpiece against a sealing or contacting surface. Once the workpiece has been engaged by the vacuum device, the workholder may be transported, retaining the workpiece, to different locations. Such workholders may also be employed to fix the workpiece for machining or fabricating operations thereon. In many vacuum devices the orifice is connected directly to the fluid sink such that air flows freely through the orifice when the vacuum pump is operating and the orifice is not covered by workpiece surface.
In multiple workpiece handling apparatus, a plurality of such workholders may be connected in parallel such that the vacuum devices of each share a common fluid sink. However, in operation, such apparatus typically experience a significant loss of vacuum pressure due to the load placed on the common fluid sink system when even a single orifice is left uncovered. To overcome this problem and provide for sufficient vacuum pressure to retain workpieces, larger capacity vacuum pumps and valve means, which permit vacuum pressure to be applied through the orifice only when there is a workpiece to be retained in the immediate vicinity, have been used separately or in combination.
However, to conserve energy and reduce production costs, it is preferable to employ low capacity vacuum pumps and only apply vacuum pressure to the orifice when a workpiece is in contact with it. Current valved orifices typically employ a valve stem extending from the workpiece contacting surface for opening the valve as the workpiece approaches the orifice. However, since the valve is opened prior to contact of the workpiece with the contacting surface, the surrounding atmosphere is permitted to flow freely through the orifice at least for a finite period of time. This may result in significant loss of vacuum pressure in the workholder system. Also, valved orifices typically apply greater localized stresses to the workpiece as it moves into engagement than non-valved vacuum orifices. These increased stresses arise where the surface area of the valve stem in contact with the workpiece is much smaller than the surface area of the workpiece contacting surface.
Another disadvantage of many valved orifices arises when the valve stem is continuously spring biased toward the workpiece. This necessitates the use of vacuum pressure high enough to retain the workpiece as well as overcome the spring bias. Spring biased valve stems also cause the above-mentioned localized stresses to be continuously applied to the workpiece. Further, where the spring bias is provided by a separate coiled spring or leaf spring element, the mechanical complexity and assembly costs of the valve are increased. Bi-stable valves per se are known, but are typically relatively complicated and unsuited for use in vacuum gripping assemblies for multiple workpieces. By "bi-stable valves", the applicant refers to valves which are normally closed but which will stay open after initial actuation without continuous application of the actuation force.
For these reasons, many current vacuum gripping assemblies are undesirable to function in workholders for thin or fragile workpieces, such as crystalline semiconductor disks and electrical circuits formed therefrom.
It is therefore an object of the present invention to provide an improved vacuum gripping means.
Another object of the present invention is the provision of a vacuum gripping assembly which exerts a minimum of localized stresses on the workpiece.
A further object of the present invention is to provide a self-actuating extractor having valve means which is normally closed but which will remain open once actuated for applying vacuum pressure to a workpiece.
Still another object of the present invention is the provision of a vacuum gripping workholder for fragile or thin workpieces using a low capacity vacuum pump.
Yet still another object of the present invention is to provide a vacuum extracting apparatus wherein vacuum pressure is applied to the gripping orifice only when the workpiece is in contact therewith.
These and other objects of the present invention are attained in the provision of a vacuum gripping assembly having base and head portions separated by a flexible conduit and bi-stable valve means for controlling the application of vacuum pressure to the head portion. The valve means is actuated by contact forces of a workpiece against the head portion which are of sufficient magnitude to compress the conduit and open the valve. Once actuated, the valve means remains open by the application of vacuum pressure on the workpiece which is positioned against the workpiece contacting surface in the head portion.
The flexible conduit serves to bias the head portion away from the base portion and conduct vacuum pressure to the head portion. This conduit may include a flexible bellows means and a rigid guide tube to provide lateral support for the bellows. The guide tube also acts as a stop to prevent excessive conduit compression resulting from too large a vacuum pressure or workpiece contact force. The workpiece contacting surface is about the periphery of the vacuum gripping orifice and of sufficient area as to prevent application of undesirable localized stresses to the workpiece. A plurality of these vacuum gripping assemblies may be connected in parallel to a common fluid sink.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.